05000317/LER-2016-001

LER-2016-001, Manual Reactor Trip Due to High Secondary Side Sodium Levels Due to a Condenser Tube Leak

Calvert Cliffs Nuclear Power Plant, Unit 1
Event date:	01-25-2016
Report date:	03-21-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3172016001R00 - NRC Website

LER 16-001-00 for Calvert Cliffs Nuclear Power Plant Unit 1, Regarding Manual Reactor Trip Due to High Secondary Side Sodium Levels Due to a Condenser Tube Leak

ML16083A363
Person / Time
Site:	Calvert Cliffs
Issue date:	03/21/2016
From:	Flaherty M D Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 16-001-00
Download: ML16083A363 (6)

I. DESCRIPTION OF EVENT:

Energy. Industry Identification System (EIIS) codes are identified in the text as [XX].

A. INITIAL CONDITIONS:

Unit 1 was operating in Mode 1 at 10 percent power with the Main Turbine [TA] offline on January 25, 2016 prior to the event.

B. EVENT:

At 0235 on January 25, 2016 Operators received a Condensate [SD] Conductivity high level alarm. Chemistry was notified and began to investigate the cause of the alarm. The Chemistry Technician reported that the Condensate Conductivity levels were rising. Concurrently Operators placed the condensate demineralizers [FDM] in service in attempt to reduce conductivity. Operators entered Abnormal Operating Procedure (AOP)-10, Abnormal Secondary Chemistry Conditions. Operators secured the circulating water pump in the when Feedwater [FJ] and Condensate sodium sample levels exceeded AOP-10 threshold limits that required a reactor trip. Following verification that Emergency Operating Procedure (EOP)-0, Post Trip Immediate Actions, safety functions were met, EOP-1, Reactor Trip, was entered for an uncomplicated reactor trip.

Subsequent investigation detdrmined there was a condenser tube leak on one of the condenser tubes in 13A Condenser [COND] Waterbox. The cracked condenser tube and several surrounding condenser tubes were plugged and the site began preparations to start-up the unit.

Unit 1 was returned to Mode 1 at 2220 on January 25, 2016 and the unit returned to full power at 2310 on January 26, 2016.

During the 2016 Unit 1 refueling outage, eddy current testing confirmed a circumferential crack on the affected tube. The apparent cause was that the crack was vibration induced.

THE EVENT:

There were no structures, systems or components inoperable at the start of the event that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

January 25, 2016 0235 Received alarm in Control Room for high level of Condensate Conductivity.

0240 Chemistry Technician reported Condensate Conductivity level rising.

0255 Entered AOP-10.

0312 Chemistry Technician reported Condensate sodium of 432 ppb; Feedwater sodium of 232 ppb.

0313 Unit 1 Manual reactor trip due to exceeding AOP-10 trip criteria.

0318 EOP-0 safety functions verified met.

0331 Implemented EOP-1 for uncomplicated reactor trip.

0429 Exited EOP-1 and implemented Operating Procedure OP-4, Plant Shutdown from Power Operation to Hot Standby.

2220 Unit 1 entered Mode 1.

January 26, 2016 2310 Unit 1 returned to full power.

E. FAILURE MODES:

The failed 13A Condenser tube is a titanium tube that was installed during the 2012 refueling outage. During initial installation of the titanium condenser tubes, vertical stakes were installed to reduce vibration of the condenser tubes however the vertical stakes do not prevent vibration in the vertical direction.

Eddy current testing during the 2016 refueling outage identified that the affected condenser tube experienced a circumferential crack around approximately 180 degrees. The apparent cause for the tube leak is that the failure was vibration induced in the unrestrained direction.

F. METHOD OF DISCOVERY:

The condenser tube leak in 13A Condenser Hotwell was self-revealing. This event is documented in the site's Corrective Action Program under IR02616491.

A. SAFETY CONSEQUENCES:

The Reactor Protection System [JD] was manually initiated in response to the condenser tube leak in 13A Condenser Hotwell in accordance with AOP-10. During performance of AOP-10, Operators started 13 Auxiliary Feedwater [BA] Pump [P] and secured both Steam Generator Feed Pumps. Although both Steam Generator [SG] Feed Pumps were secured to reduce sodium buildup in the steam generators, the Main Feedwater system remained functional.

Additionally, Operators shut the Main Steam [SB] Isolation Valves [ISV] to protect the condenser from overpressure. This step, although listed in the abnormal operating procedure, was not required as sufficient condenser vacuum was still being maintained by the remaining Circulating Water [KE] Pumps. The Main Steam Isolation Valves were subsequently re-opened.

The safety consequence of this event was the introduction of sodium into the Condensate and Feedwater systems and ultimately into the Steam Generator. If left uncorrected, the high sodium conditions would have a detrimental effect on steam generator tube integrity. The condenser tube leak resulted in a manual reactor trip to be initiated once sodium levels exceeded AOP-10 allowed levels. Operators continued actions to restore steam generator secondary side water chemistry to acceptable levels.

This event satisfies the criteria in NUREG-1022, Revision 3, for system actuations of the Reactor Protection system and the Auxiliary Feedwater system. Therefore, this event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A). An immediate event notification report (51683) was also made pursuant to 10 CFR 50.72(b)(2)(iv)(B) for reactor protection system actuation while critical and 10 CFR 50.72(b)(3)(iv)(A) for the manual actuation of the auxiliary feedwater system.

B. CORRECTIVE ACTIONS:

The failed condenser tube and several adjacent tubes were plugged prior to returning the unit to power operations.

During Unit l's 2016 refueling outage eddy current testing confirmed a through-wall circumferential crack of around 180 degrees on the affected condenser tube. The apparent cause is that the condenser tube failure was vibration induced. As part of the extent of condition investigation the normal number of condenser tubes that were eddy current tested in 13A Condenser during a refueling outage was expanded. As a result four other condenser tubes in 13A Condenser were found to have indications of wear at tube stake locations and were subsequently plugged. Radial stakes were then installed during the refueling outage to reduce movement in the x and y direction. The site conducted the normal amount of eddy current testing on the remaining Unit 1 condensers with no similar issues detected.

Following the refueling outage, a causal analysis will be performed to determine the cause of the tube leak. If additional information is subsequently developed that would significantly affect the understanding of this event, a supplemental licensee event report will be submitted.

III. PREVIOUS SIMILAR EVENTS:

A review of Calvert Cliffs' events over the past several years was performed and no similar instance was found.

A. COMPONENT INFORMATION:

COMPONENT IEEE 803 IEEE 805

FUNCTION ID SYSTEM ID

13A Condenser Tube COND SG The 13A Condenser tubes were fabricated by Valtimet.